Frequency hopping spread spectrum communication system

ABSTRACT

A method of operating a frequency hopping spread spectrum, preferably a Bluetooth piconet, comprising a central node and dependent nodes which communicate over a time division duplexed, frequency hopping channel, alternate time-wise frequency/time slots being allocated for central node and dependent node transmission, wherein a first of said dependent nodes is not permitted to transmit in a frequency/time slot which immediately succeeds, time-wise, a frequency/time slot in which the central node transmitted to another of said dependent nodes, comprising the steps of: the central node maintaining a black-list of worse-performing frequency bands in the channel, and transmitting a dummy packet in a frequency/time slot immediately preceding, time-wise, a frequency/time slot allocated for possible dependent node transmission at a frequency band which is black-listed.

The present invention relates to a frequency hopping spread spectrumcommunication system.

In most countries, the part of the spectrum commonly known as theIndustry Scientific Medicine or ISM band (in the region of 2.4 GHz) islargely unregulated, meaning that no licence is needed to makeelectromagnetic transmissions in this band.

In this unruly part of the spectrum, frequency hopping spread spectrumsystems have been found to have good performance. In these systems, thecarrier frequency of a modulated information signal changes or hopsperiodically to another (or possibly the same) frequency of a set ofpossible frequencies called the hopset. The hopping sequence is governedby the spreading code. FIG. 1 shows the time/frequency occupancy of anexemplary communication between two nodes of a frequency hopping spreadspectrum system. Thus, because the frequency hopping spread spectrumsystem is continually hopping between parts of the spectrum, the effectof narrow band interference in a particular region is limited.

The present invention is concerned with the operation of a frequencyhopping spread spectrum communication system in the presence of a“persistent interferer” such as, for example, a microwave oven or a WLANnetwork operating at a fixed region of the spectrum.

Persistent interferers present two distinct problems to this kind ofsystem.

(i) “System Performance”

Although the use of a frequency hopping spread spectrum system per selimits the degradation in system performance caused by a persistentinterferer, the effect on system performance can be significant,especially in the presence of several persistent interferers.

(ii) “System Compatibility”

WLANs often transmit data in large packets. The presence of a nearbyfrequency hopping spread spectrum regularly hopping into the region ofthe spectrum used by the WLAN during the transmission of a large packetcan have a devastating effect on the WLAN's performance.

With this background in mind, according to the invention, there may beprovided a method of operating a frequency hopping spread spectrumcomprising a central node and dependent nodes which communicate over atime division duplexed, frequency hopping channel, alternate time-wisefrequency/time slots being allocated for central node and dependent nodetransmission, wherein a first of said dependent nodes is not permittedto transmit in a frequency/time slot which immediately succeeds,time-wise, a frequency/time slot in which the central node transmittedto another of said dependent nodes, comprising the steps of:

-   -   the central node maintaining a black-list of worse-performing        frequency bands in the channel, and transmitting a dummy packet        in a frequency/time slot immediately preceding, time-wise, a        frequency/time-slot allocated for possible dependent node        transmission at a frequency band which is black-listed.

By virtue of these features, the central node is able to preventativelyforestall the use of the worse-performing frequency bands without anyadditional dedicated signaling protocols. By preventing the use of theworse-performing frequency bands, the method of the present inventionrepresents a much less disruptive influence on neighbouring ISM-bandsystems.

Exemplary embodiments of the invention are hereinafter described withreference to the accompanying drawings, in which:

FIG. 1 shows the time/frequency occupancy of a frequency hopping spreadspectrum communication system:

FIG. 2 shows a frequency hopping spread spectrum communication systemhaving 5 nodes;

FIG. 3 shows a hardware block diagram for a node of FIG. 2;

FIG. 4 show a diagram illustrating the operation of the system;

FIG. 5 shows the time/frequency occupancy of the channel of the FIG. 2system in the presence of a persistent interferer; and

FIG. 6 shows time/frequency occupancy of the channel of FIG. 2 systemwhere 2 black-listed frequency band are removed from the hop set.

FIG. 2 shows a frequency hopping spread spectrum system, generallydesignated 5, operating in the ISM band. The system 5 comprises fivenodes arranged as a piconet, in which a node 10 serves as the masternode and the other four nodes serve as slave nodes 12 a, 12 b, 12 c and12 d. These devices are lower power RF devices, preferably operating inaccordance with the Bluetooth protocol. Within the system 5,communication takes place, bidirectionally, between the master node 10and any of the slave nodes 12 a, 12 b, 12 c and 12 d. No communicationtakes place directly between the slave nodes themselves.

Each node 10,12 is identical having the same hardware and the samesoftware enabling it to be operable to act as a master node or a slavefor a given network, or possibly acting as the master node for a firstnetwork while simultaneously acting as a slave node for a secondnetwork.

In more detail, referring to FIG. 3, each node 10,12 comprises atransmitter 20, a receiver 30 and a control processor 40. Thetransmitter 20 comprises a baseband modulator 21 to which baseband datafor transmission is supplied by the control processor 40. The basebandmodulator produces a modulated data signal and feeds it to anupconverter 22. The upconverter 22 shifts the modulated data signal to afrequency dictated by a frequency synthesiser 23 for transmission by anantenna 25. The output frequency of the frequency synthesiser 23 iscontrolled by a spreading code output by a code generator 24. Thereceiver 30 comprises a complementary architecture. A downconverter 32shifts the signal received via an antenna 35 to a lower frequency,governed by the output frequency of a frequency synthesiser 33, and feedthis frequency-shifted signal to a demodulator 31 for demodulation tobaseband data. The output frequency of the frequency snythesiser iscontrolled by a locally generated spreading code output from a codegenerator 34. Synchronization and tracking circuitry 36 ensure that thelocally generated carrier synchronises. sufficiently well to thereceived carrier so that correct despreading of the received signal ispossible.

FIG. 4 show a diagram illustrating the operation of the system. A timeline 108 shows the system in various phases of operation: initialisation110, evaluation. 120, and configuration 130. The time in which theevaluation phase takes place is referred to as the evaluation interval,T_(eval), and the time in which the evaluation phase and theconfiguration phase take place is referred to as an epoch, T_(epoch).

On initialisation, each slave node 12, as it joins the piconet, is givena local piconet address, by which the master node addresses the slavenode, and is synchronised to follow a hop sequence F within thefrequency range comprising frequency bands F1 to F8, the portion of thehop sequence shown in FIG. 1 being F1, F5, F3, F2, F7, F2, F8, F6, F3and the corresponding time/frequency slot being labeled 100 a-i. Eachtime slot in the hop sequence is alternately reserved for transmissionby the central node 10 (D slots) and transmission by the dependent nodes(U slots). In Bluetooth, the maximum number of slaves in a singlepiconet is 7. System parameters, for example, T_(eval) and T_(epoch) arealso set in this initialisation phase.

After initialisation, for the evaluation interval, T_(eval),communication between each slave node 12 and the master node 10 takesplace as illustrated by flowchart (i) of FIG. 4,

If a slave node, say node 12 a, wants to transmit a packet to the masternode 10, it waits until the next available U time/frequency slot, forexample, time/frequency slot 100 a (shown in FIG. 1), and makes itstransmission during this time/frequency slot 100 a, step 122, and waitsfor an acknowledgement from the master node 10, step 124, on the next Dslot, namely time/frequency slot 100 b (shown in FIG. 1).

If an acknowledgement (ACK) is not received in time/frequency slot 100b, then the slave node 12 a assumes that the packet transmitted intime/frequency slot 100 a was not properly received by the master node12 (step 126). The failure of the master node to receive the incomingpacket could have been because of collision with an attempted packettransmission by another slave node 12 b-d in the same system, collisionwith a neighbouring similar system having a different master node, orinterference from the previously-mentioned persistent interferers such amicrowave or a WLAN network.

The slave node 12 a maintains a record for the evaluation interval,T_(eval), of how many times it has tried to make a transmission on eachfrequency band F1-8, T_(i), and how many of those times the transmissionwas successful, NS_(i) and, from this information, calculates, at step128, a local interference indices I_(Fi) (where in this exemplary systemi=1 to 8 because there are eight frequency bands in the channel). Inthis case, the slave node 12 a calculates a new value for I_(F1) becausethe packet transmission was attempted on U time slot 100 a, whichoccupies frequency band F1, according to the relationshipI _(F1)=(T ₁ −NS ₁)/T ₁   (1)

This process is repeated every time the slave node 12 a fails tosuccessfully transmit a packet or to transmit a packet for the firsttime. Each slave node 12 independently carries out the same process.

In this way, each slave node 12 builds up a picture over the evaluationinterval, T_(eval), of its own local view of how prone to interferenceeach frequency band, F1 to F8, in the channel is. This picture isencapsulated in the interference indices I_(Fi) stored at each slavenode 12.

At the end of the interval, the system moves into the configurationphase 130 as illustrated by flowchart (ii) in FIG. 4. The master node 10broadcasts on a b time/frequency slot a command addressed to a selectednode to transmit its local interference indices I_(Fi) to the masternode on the next U slot (step 132). At step 134, the addressed slave 12receives the request and, at step 136, transmits the interferenceindices I_(Fi). At steps 138,140 and 132, if the master node 10 does notproperly receive the interference indices I_(Fi), it re-makes itsrequest on the next D time/frequency slot. The master node 10 repeatsthis interrogation process until it has successfully received the localinterference indices for each dependent node (step 142).

With the interference indices I_(Fi) from each slave node 12, the masternode 10 calculates the system-aggregate performance of each frequencyband Fi in the channel, in particular the system-aggregate probabilityof error free transmission over the previous interval P_(t) (step 144),P _(t)(Fi)=Σ(I _(Fi) /n)   (2)where n=the number of slave nodes.

Based on this, at step 146, the master node 10 identifies theworst-performing frequency bands by comparing their respective Pt overthe last evaluation interval 120 and creates a black list of the twoworst-performing frequencies.

At the end of the current epoch, i.e. after the configuration phase, thesystem again enters the evaluation phase 120. Now, armed with theknowledge of which frequencies are worst-performing the master nodes 10,12 again follow the hop sequence F, but (i) the master node 10 omits totransmit on the two black-listed frequency bands because it knows thatthey are known to be poorly performing, and (ii) the master node 10transmits a dummy packet in the frequency/time slot immediatelypreceding, time-wise, a frequency/time slot which is transmitted on afrequency band which has been black-listed. The dummy packet isaddressed to a slave node piconet address for which there is no slavenode currently assigned. By sending the dummy packet, the master node 10is telling the other real, slave nodes 12 that the next U slot isreserved for the acknowledgment of the addressed (dummy) slave node 12,and in so doing, by indirect means, prevents transmission on theblack-listed frequency band. In the case when the piconet is full andthere are 7 slave nodes, the master node 12 pre-emptively puts one ofthe slave nodes into park mode to free up a dummy piconet slave address.In park mode, a slave node is merely maintaining synchronisation withthe piconet and needs to be re-activated before it can againcommunication with the master node.

The system operation proceeds as before, except during the second andsubsequent configuration phases 130, the parameters P_(t)(Fi) areadjusted according to the parameter α (where 0≦α≦1) is and the value ofthe P_(t) calculated during the previous configuration phase, P_(T-1).P _(t)(Fi)=α.P _(t)(Fi)+(1−α).P _(t-1)(Fi)   (3)

This modification of P_(t)(Fi) has the effect, to an extent governed bythe value of α, of making P_(t) reflect not only the frequency bandsperformance over the evaluation interval but also the historicperformance over previous evaluation intervals.

If the situation is now considered where the system 5 is being used inthe vicinity of a WLAN. This network sporadically transmits relativelylong packets of information in an area of the spectrum which fallswithin the frequency bands used by the system 5. An example of theinterference of this neighbouring WLAN is shown in FIG. 5 and denoted102. It will be appreciated that for the portion of the hopping sequenceF shown in FIG. 5, the F7 and F8 frequency bands are completely swampedby the long packet transmission of the WLAN. By comparison with FIG. 1,we can see that frequency/time slots 100 e and 100 g have been rendereduseless. Although only a small portion of the hopping sequence isvisible in FIG. 5 if we assume that transmissions of the WLAN havedisrupted F7 and F8 for much of an evaluation interval, then it will beappreciated that any slave node 12 which tries to make use of F7, F8 isnot likely to meet with much success. As a result, the slave nodes allcalculate during the evaluation phase 120 local interference indices forI_(F7) and I_(F8), according to equation (1) above, which areconsiderably smaller than for the indices of the other frequency bandsF1-6. Accordingly, during the configuration phase 130, the master node10 collects the local interference indices from each slave node 12 andcalculates a system-aggregate probability of error-free transmissionP_(t) for each frequency F1 to F8 according to equation (2) and adjustsit according to historic information as per equation (3), the masternode identifies frequency bands F7 and F8 as the worst performing, i.e.most interfered with, frequency bands and so places them on a black list(step 146). At the start of the next evaluation phase 120, with thisknowledge of which frequency bands have been black listed, the masternode 10 refrains from transmitting on any frequency/time slots whichfall in a black-listed frequency band. With reference to FIG. 1, it willbe appreciated that there are no D slots which are transmitted on F7 orF8 in the example shown. However, there are two U slots 100 e,transmitted on F7, and 100 g transmitted on F8. In order to forestalltransmission on these frequency/time slots, the master node 10 transmitsto a dummy slave node, say a fifth slave node 12 e, which is not shownin FIG. 1 because it does not exist. By virtue of this transmission, allthe slave nodes 12 a-d are not permitted to transmit on the immediatelysucceeding U slots, 100 e and 100 g. In this way, the system 5 aims toavoid in advance the RF hot spots in the local environment as shown inFIG. 6. This not only improves its own system performance, but alsomakes the system 5 far more sociable in RF terms to neighbouring systemse.g. a WLAN.

Once a frequency band has been black-listed it is no longer in use bythe system and hence no fresh interference index is being calculated bythe slave nodes. Therefore, the master node 10, when at step 146 it isdeciding upon the black-listed channels for the next epoch, uses thevalue of the interference indices, which the currently black-listedchannels had immediately before they were black listed scaled by β^(x)where β<1 and x is the number of epochs that the frequency band has beenon the black list, as the basis of comparison with the newly-gatheredinterference indices from the unblack-listed frequency bands.

It will be appreciated that the selection of the system parameters α andβ have a great influence on under what circumstances and for how long agiven frequency band is black listed. For example, the greater the valueof α, the greater the weighting given to the environment in only theprevious evaluation phase 120. Whereas, if α has a small value, thengreater weighting is given to the conditions in the environment in thepast. Regarding β, if β is small, then the black-listed channels have agreater chance of being quickly taken off the black list as comparedwith when β is close to 1.

It will be appreciated that for ease of description and for concision asimplified embodiment has been described. For example, the number offrequency bands in the channel was 8. But in a practical system thereare likely to be many more frequency bands. According to the FCCregulations, a frequency hopping system in accordance with thisinvention operating in the ISM band must hop onto 75 out of the possible79 frequency bands available. Although in the described embodiment, twofrequencies bands are placed onto the black list. In practice, thisnumber may be dictated or at least constrained by governmentalregulations.

In the described embodiment of the invention, the master node 10collates the interference indices I_(Fi) by interrogating the slavenodes 12 in turn. In another embodiment, the slave nodes could sent thisinformation after a predetermined time. In this case of course, thetiming for the slave nodes to dispatch this information to the masternode needs to be such that all the slave nodes access to the same Uslot, to prevent excessive collisions between the slave nodes.

In the described embodiment of the invention, the evaluation period foreach frequency band, but in other embodiments, the evaluation period foreach node can be different, even substantially different.

1. A method of operating a frequency hopping spread spectrum comprisinga central node and dependent nodes which communicate over a timedivision duplexed, frequency hopping channel, alternate time-wisefrequency/time slots being allocated for central node and dependent nodetransmission, wherein a first of said dependent nodes is not permittedto transmit in a frequency/time slot which immediately succeeds,time-wise, a frequency/time slot in which the central node transmittedto another of said dependent nodes, comprising the steps of: the centralnode maintaining a black-list of worse-performing frequency bands in thechannel, and transmitting a dummy packet in a frequency/time slotimmediately preceding, time-wise, a frequency/time slot allocated forpossible dependent node transmission at a frequency band which isblack-listed.
 2. A method as in claim 1, wherein the central noderefrains from transmitting on a black-listed frequency/time slot.
 3. ABluetooth node comprising means for maintaining a black-list of worseperforming frequency bands, and means for transmitting a dummy packet ina frequency/time slot immediately preceding, time-wise, a frequency/timeslot allocated for possible slave node transmission at a frequency bandwhich is black-listed.
 4. A Bluetooth node as in claim 3, comprisingmeans for refraining from transmitting on a given frequency/time slot onthe basis of the black-list.